To mitigate the spread of COVID-19, universities in Japan shifted from face-to-face to online classes, which might have reduced social interaction and increased psychiatric problems among students. A self-report questionnaire was administered to fourth-year medical students in Tokyo in May 2021, during the fourth wave of the COVID-19 outbreak, to examine the association between the frequency of conversations and suicidal thoughts. The questionnaire assessed the frequency of conversations and, using part of the Mini International Neuropsychiatric Interview, suicidal ideation. Of the 113 students, 98 (86.7%) responded, of whom 20 (20.4%) had suicidal ideation. Poisson regression analysis revealed that those with less than 1 conversation per week and no conversations at all had a significantly higher risk of suicidal ideation than those with 3 conversations per week or more, after adjusting for personality, family relationship, income level, living alone, number of friends, gender, and age. These results indicate that less frequent conversations increased the risk of suicidal ideation among medical students. Mental health support for students needs to be strengthened if universities suspend face-to-face classes during a pandemic.